When a voltage is applied on an organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), holes and electrons are respectively injected into an emitting layer from an anode and a cathode. In the emitting layer, the injected holes and electrons are recombined to form excitons. Herein, singlet excitons and triplet excitons are formed at a ratio of 25%:75% according to electron spins statistics. In a classification according to emission principle, in a fluorescent organic EL device which uses emission caused by singlet excitons, the limit value of an internal quantum efficiency is believed to be 25%. On the other hand, in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
In a typical organic EL device, the most suitable device design has been made depending on fluorescent and phosphorescent emission mechanism. Particularly, when a fluorescent device technique is simply used for designing the phosphorescent organic EL device, it has been known that a highly efficient phosphorescent organic EL device cannot be obtained because of a luminescence property of the phosphorescent organic EL device. The reasons are generally considered as follows.
First of all, since the phosphorescent emission is generated using triplet excitons, an energy gap of a compound for the emitting layer must be large. This is because a value of singlet energy (i.e., an energy gap between energy in the lowest singlet state and energy in the ground state) of a compound is typically larger than a value of triplet energy (i.e., an energy gap between energy in the lowest triplet state and energy in the ground state) of the compound.
Accordingly, in order to efficiently trap triplet energy of a phosphorescent dopant material in the device, a host material having larger triplet energy than that of the phosphorescent dopant material needs to be used in the emitting layer. Moreover, when an electron transporting layer and a hole transporting layer are provided adjacent to the emitting layer, a compound used as the electron transporting layer and the hole transporting layer need to have a larger triplet energy than that of the phosphorescent dopant material. Thus, according to the designing idea of the typical organic EL device, a compound having a larger energy gap than that of a compound used as a fluorescent organic EL device is used for producing a phosphorescent organic EL device. As a result, a drive voltage of the overall phosphorescent organic EL device increases.
Although a hydrocarbon compound exhibiting a high oxidation resistance and a high reduction resistance is useful for the fluorescent device, the hydrocarbon compound has a broad π electron cloud to render the energy gap small. For this reason, such a hydrocarbon compound is unlikely to be selected as the phosphorescent organic EL device but an organic compound containing a hetero atom such as oxygen and nitrogen is selected. However, the phosphorescent organic EL device in which the organic compound containing a hetero atom is used in an emitting layer exhibits a shorter lifetime than that of the fluorescent organic EL device.
Moreover, device performance of the phosphorescent organic EL device is greatly affected by an exciton relaxation rate of triplet excitons much longer than that of singlet excitons in the phosphorescent dopant material.
In other words, with respect to emission from the singlet excitons, since a relaxation rate leading to emission is so fast that the singlet excitons are unlikely to diffuse to the neighboring layers of the emitting layer (e.g., the hole transporting layer and the electron transporting layer), efficient emission is expected.
On the other hand, emission from the triplet excitons is based on forbidden spin transition and a relaxation rate is slow. Accordingly, the triplet excitons are likely to diffuse to the neighboring layers, so that the triplet excitons are thermally energy-deactivated unless the phosphorescent dopant material is a specific phosphorescent compound. In short, in the phosphorescent organic EL device, control of the recombination region of the electrons and the holes is more important as compared with control thereof in the fluorescent organic EL device.
For the above reasons, advancement of the phosphorescent organic EL device requires material selection and device design different from those of the fluorescent organic EL device.
A carbazole derivative is typically known as a compound used for the phosphorescent organic EL device. The carbazole derivative exhibits a high triplet energy and has a carbazole skeleton known as a main skeleton of a hole transporting material. The carbazole derivative is used as a useful phosphorescent host material.
Patent Literature 1 (JP-A-2004-217557) and Patent Literature 2 disclose that a compound obtained by bonding two carbazole rings through a linking group is used as a material for an organic EL device. Patent Literature 3 (JP-A-2006-199679) discloses that an N,N-carbazole compound in which two carbazole rings are bonded to each other at respective ninth positions (N position) through a linking group Z into which a nitrogen-containing heterocyclic group is introduced is used as a material for an organic EL device.
However, improvement in a luminous efficiency of the organic EL device is still demanded and development of a compound capable of improving a luminous efficiency and a material for an organic EL device containing the compound is desired.